1. Field of the Invention
This invention relates to the field of optical holography and, more specifically, to an apparatus for providing a stable exposure station and mechanism for transporting non-perforated film in uniform steps without backlash and without inducing film strain for use in the making of integral holograms.
2. Description of the Related Art
Methods of making holographic stereograms are well known in the holographic art. Generally speaking, such stereograms are synthesized from a series of two dimensional transparencies, each transparency corresponding to a differing viewpoint of a subject. During reconstruction, the resulting composite hologram gives a three dimensional representation of the subject, which depending on the method of manufacture, is viewable with either laser or white light illumination, and may provide either full parallax or horizontal-only parallax.
A survey of various type of holographic stereograms is given in Benton, S, "Survey of Holographic Stereograms," in Processing and Display of Three-Dimensional Data. SPIE 367:15 (1983).
Cylindrical rainbow holographic stereograms, viewable in white light, were invented by Lloyd Cross in 1973 and are known in the literature as Cross-type or multiplex holograms. Several improvements of the original Cross-type hologram have been reported in the literature. These include methods for reducing distortion in the holograms, various alternative optical systems for circumventing the cumbersome liquid-filled optics of the original design, and methods to attain flat playback without distortion.
The multiplex hologram is formed by sequentially recording a number of vertically oriented strip holograms laid side-by-side on a photosensitive emulsion. In its original form the multiplex hologram was recorded on a 120-degree section of a 16" diameter cylinder. Holographic film was taped to a 120-degree section of a plastic cylinder with the concave surface of the section facing the optical system of the apparatus. The section was then mounted on a rotary platform with indexable motor means. By indexing the motor means in uniform steps, a sequence of strip holograms were then recorded. An advantage of the approach was that film was kept relatively stable during the holographic exposures and was not subjected to strain. A disadvantage of the approach was that in order to construct a full 360-degree hologram of the subject, three individual 120-degree multiplex holograms had to be constructed and spliced together to form the 360-degree composite hologram. Alignment of the edges of the three holograms was critical, and if not properly done, resulted in double images at the splices. From an aesthetic point of view, the splices were visually distracting even if the problem of the double images was eliminated. Later efforts have centered on methods of recording both 360-degree format and flat format multiplex holograms on a single length of film.
For purposes of the present invention and to avoid confusion with the various terms employed in the literature, the term "integral hologram" shall be used to refer to a transmission-type white-light viewable multiplex holographic stereogram composed of a plurality of vertically-oriented strip holograms recorded sequentially on a continuous length of film.
Integral holograms are all made utilizing a step-and-repeat process. The process generally comprises the steps or exposing a strip hologram, advancing the holographic film for the next recording, advancing the transparency-film transport, and waiting for a pre-determined stabilization interval before repeating the process. As many as 2100 strip holograms are recorded in making a 360-degree integral hologram.
The film used until recently for making this type or integral hologram, Kodak SO-173, was stocked in 91/2" widths and available in other widths by special order. Currently the film preferred by the inventors is Agfa 8E75HD which is available in 10" widths and is non-perforated. The difficulty of stabilizing such large format film during strip recording of an integral hologram and not subjecting it to strain can be appreciated by reference to early papers in the literature that suggested stabilization times averaging 15 minutes before recording a single unitary hologram on a Kodak 649-F plate.
Since each strip hologram in the integral hologram is viewed in reconstruction in relation to its strip-neighbor, the percent modulation of each strip hologram should ideally be equal. Deviation from uniform modulation results in a "picket-fence" effect in the final hologram which is visually distracting.
In order to eliminate variations in modulation in an otherwise stable holographic system, the present invention has been developed to minimize strain on the film during the transport and recording process and to provide a stable exposure station so as to require a minimum stabilization time between recording of the strip holograms.
To the inventors' knowledge, prior art exposure stages and film transports for integral holographic applications have employed drive rollers in various configurations to move the film during the interval between the recording of each successive strip hologram. In such devices, if drive rollers are fixed relative to the holographic film and driven independently of means to drive the supply and take-up reels, any slight misalignment of the drive rollers relative to the vertical plane of the film will cause the film to move vertically as it is advanced. Owing to the resistance the drive rollers exert on the film, the film will eventually bind in the film guides.
A more complicated drive roller assembly could obviate this problem by retracting from the film after each strip hologram is recorded. However, such retraction could allow the film to readjust its position in the absence of drive roller friction thus causing unpredictably un-equal strip widths to be recorded.
In U.S. Pat. No. 4,411,489 a film exposure stage and film transport is disclosed consisting of a supply roller, a take-up roller, and a rotatable cylindrical platen. As stated in the disclosure, the take-up roller and platen are driven by a conventional drive system. Although not indicated, supply and take-up rollers would require tensioning means to maintain the film against the cylindrical platen which could induce strain in the film during the recording process.
Amesbury reports a precision film transport for incremental or continuous transport of large format film past a film gate for use in a laser scanning system, [Amesbury, M., "Large Format Film Transport" in Data Extraction and Classification from Film. SPIE 117:103 (1977) P 103.]. The transport is capable of incremental steps from 0.01" to 10" and relies on a servo controlled tensioning system for transport of the film. This approach, while appropriate for the intended use, would be counter-indicated in a holographic transport due to the strain the tensioning system would impart to the film.
The present invention provides a viable alternative to the prior art which is not subject to the disadvantages of the different systems noted above. Rather than employing drive rollers to move the holographic film, a step-and-repeat mechanism is disclosed which incorporates a drive device to advance the holographic film. The drive device of the present invention yields repeatably spaced strip holograms even for strip widths of the order of 0.005".
The drive device coupled to an anti-rise device in the present invention compensates for any slight tendency for the film to rise during film movement between successive recording of the strip holograms. The drive device exerts minimal force on the bottom edge of the film and thereby minimizes strain on the holographic film.
In one embodiment, the drive device is vacuum driven by piston-cylinder means. A vacuum-platen exposure stage is provided which keeps the film stable during exposure. The vacuum-platen is operationally coupled to the drive device which allows the device to release the film between successive film advances without the backlash and concomitant wear associated with the use of motors, bearings, and the like. Advantageously, a single vacuum system activates both the drive device and the vacuum-platen.
The width of each strip hologram in a given integral hologram is accurately adjustable by a micrometer which varies the stroke of the piston advance mechanism.
Rather than employing supply and take-up reels with drive motors, the present invention employs passive drums to contain the film at the supply and take-up side of the transport. Advantageously, this allows a length of integral hologram to be recorded which is at least twice the length currently made, without requiring additional, stress inducing, coupled supply and take-up drive motors.